excessive_precision formats large negative exponent wrong

[dtolnay]

fn main() {
    let _ = 2.225_073_858_507_201_1e-308_f64;
}

warning: float has excessive precision
 --> src/main.rs:2:13
  |
2 |     let _ = 2.225_073_858_507_201_1e-308_f64;
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing the type or truncating it to: `2.225_073_858_507_201e-_308_f64`
  |
  = note: `#[warn(clippy::excessive_precision)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#excessive_precision

Clippy's suggested diff is:

-     let _ = 2.225_073_858_507_201_1e-308_f64;
+     let _ = 2.225_073_858_507_201e-_308_f64;

which has the floating point exponent written as e-_308 rather than the more readable e-308.

I'm guessing something in the underscore insertion is indirectly based on (-308).to_string().len() > 3.

Meta

Rust version (rustc -Vv):

rustc 1.57.0-nightly (aa7aca3b9 2021-09-30)
binary: rustc
commit-hash: aa7aca3b954131720df725e70d12e902eb3be1de
commit-date: 2021-09-30
host: x86_64-unknown-linux-gnu
release: 1.57.0-nightly
LLVM version: 13.0.0